Field of the Invention and Related Art
The present invention relates to a color toner for developing electrostatically charged images in the fields of electrophotography, electrostatic recording, and electrostatic printing, and, in particular to a color toner having excellent color reproducibility in color images and excellent offset durability, a developer using the color toner for developing electrostatic images, an image forming apparatus, color image forming method and a process for producing a color toner.
In recent years, full-color copying machines have increasingly attracted attention, and especially digitalized full-color copying machines.
In a color image forming process of full-color electrophotography, the color is generally reproduced by using three colors of yellow, magenta, and cyan, or optionally by adding black.
A general color image forming method is as follows; first, rays of light from a document form an electrostatic latent image on a photoconductive layer through color separation light transmission filters which have complementary colors to the respective toners' color. Next, the toner is held on a toner image supporting member through developing and transfer steps. The steps are repeated several times while adjusting registration to overlap toner images on the same supporting member. A final full-color image can be obtained by a fixation step.
The fixation characteristics of the color toners are significantly important in color electrophotography which requires a plurality of developing steps and overlapping of various color toner layers on the same supporting member during the fixation steps.
The fixed color toners require appropriate gloss, and any irregular reflections due to the toner particles must be reduced as much as possible. Further, the color toners require sufficient transparency that any upper toner layer does not inhibit or interfere with the lower toner layers, each having a different tonality.
The present inventors have disclosed combinations of novel binder resins and coloring agents for color toners satisfying the above demands in Japanese Patent Laid-Open No. 50-62442, 51-144625, and 59-57256. The disclosed color toners have considerable sharp melting characteristics. Further, in the combination with silicone rubber rollers enabling the coating of silicone oils, the toners can be almost completely melting during the fixing step and still show desirable gloss and color reproducibility.
These effects demonstrate that, for the fixing characteristics of toners, the viscosity factor is more important than the elasticity factor in the viscoelasticity of the binder resins. Namely, the toners preferentially show the behavior as the viscosity factor during heating so that the hot melt characteristics are enhanced and gloss appears in the toners.
A binder resin design, which weighs such a viscosity factor, necessarily causes a decreased intermolecular cohesive force during the hot melt process and increased toner scale on the hot rollers during passing through the fixation apparatus. These problems easily bring about high temperature offset.
When using silicone rubber rollers as the fixing rollers particularly, the high temperature offset easily occurs during repeated operation due to the decreased releasing property inherent in the silicone rubber rollers independently of the coating of a release agent. At the initial stage when using the silicone rubber rollers, the releasing property can be maintained to some extent due to the silicone oil impregnated in the silicone rubber and the smooth, clean surface of the rollers. However, during continuous color copy operation of a large image size and significantly high toner holding content on the supporting member, such as an ordinary paper compared with monochrome copy, the oil in the silicone rubber will become exhausted and the roller surface will roughen so that the releasing property of the roller will gradually decrease. The deterioration speed of the roller is almost several times as fast as that in monochrome copying.
Moreover, the toners themselves have less elasticity as mentioned above resulting in decreased offset resistance of the toners. Thus, the high temperature offset is observed after only a few thousand to tens of thousand copies; coatings and scales of the toners form on the fixing roller surface; and the upper toner layers are peeled off from the imaged surface during passage through the nip of the hot roller.
Various attempts have been made to solve or decrease the above problems on toners and further improvements are required. For example, a release agent such as low molecular weight polyethylene, polypropylene, wax, and higher fatty acids is added to the toner in order to increase its releasing property as described in Japanese Patent Laid-Open No. 55-60960, 57-208559, 58-11953, 58-14144, and 60-123852. Although these methods are effective for preventing offset, a high content of release agent unsatisfactorily decreases the miscibility with the biding resin, resulting in the following harmful effects; loss in transparency of projected color image by OHP (over-head projector), unstable electrostatic charge, and decreased durability.
In Japanese Patent Laid-Open Nos. 47-12334, 57-37353, and 57-208559, toners containing non-linear polyester copolymers as a binder are proposed. The polyester copolymers are obtained from monomer components including etherized bisphenol monomers, dicarboxylic acid monomers, trivalent or higher polyhydric alcohol monomers and/or trivalent or higher polycarboxylic acid monomers. Such prior art seeks to prevent offset by containing the polyester binder, which is obtained by crosslinking polyester comprising etherized bisphenol and dicarboxylic acid monomers with a large quantity of the polyhydric alcohol and/or polycarboxylic acid monomers, in the toner. However, those toners have somewhat high softening temperatures and do not show satisfactory fixing characteristics at a low temperature. Further, although the high temperature offset reaches a practical level for full-color copying, mixing property and reproducibility of the colors based on the overlap of the full-color toners are unsatisfactory due to poor fixation and melt characteristics. In Japanese Patent Laid-Open Nos. 57-109825, 62-78568, and 62-78569, as well as Japanese Patent Laid-Open Nos. 59-7960 and 59-29256 by the present inventors, the toners containing polyester binders are disclosed, in which the polyester is a non-linear copolymer obtained from an etherized bisphenol monomer, a dicarboxylic acid monomer substituted with higher aliphatic hydrocarbon and another dicarboxylic acid monomer, a trivalent or higher polyalcohol monomer and/or a trivalent or higher polycarboxylic acid monomer. The polyester has a side chain having a saturated or unsaturated aliphatic hydrocarbon group of 3 to 22 carbon atoms. These polyester binders are intended for use in high speed copying machines, and to meet the weight elasticity factor of the viscoelasticity of the resin, in contrast to the above viscosity factor weighted polyester, resulting in increased elasticity and drastically decreased high temperature offset to the roller. Here, the pressure and temperature of the hot roller are raised as much as possible during the fixation, and toners are squeezed into the spaces between fibers of the transferred sheet in a semi-melted state, so as to be fixed at a high pressure and temperature.
Thus, continuous coating formation and smooth surface formation due to melting of the toner layers, which are essential for color copying, are practically impossible. As a result, fixed toners exist in a particulate state on the transfer paper, and the obtained color image is subdued and has low color saturation. In the image on the transparent sheet, the light scatters and diffuses on the surface of the toner particles impractically reducing light transmittance.
The present inventors have proposed novel polyester resins having excellent high temperature offset resistance and applicable to color copying in Japanese Patent Laid-Open Nos. 2-73366 and 1-224776. These resins have excellent properties compared with conventional resins for the color toners. However, the offset prevention to the fixing rollers is effective only for 20 to 50 thousand times of repeated operation. Considering that, in the monochrome toner, printing durability and offset resistance for a few hundred thousand copies are required in spite of the conventional life span of somewhat more than one hundred thousand copies, these properties in the color toner are desired to be further improved. Because these polyester resins have a great difference in electrostatic chargeability between a low temperature-humidity atmosphere and a high temperature-humidity atmosphere, in color imaging after repeated copying, the image density is somewhat deduced at a low humidity atmosphere, and toner scattering and fog sometimes occur at a high humidity atmosphere.
Polyester resins are disclosed in Japanese Patent Laid-Open Nos. 62-195676, 62-195678 and 62-195680, in which the ratio of hydroxyl number to acid number are limited. These polyester resins are intended for high speed fixing, and the color toners using such polyester resins do not provide satisfactory color mixing properties according to the present inventors.
As a marked characteristic of color copying, harmonization of at least three colors and preferably four color toners, is essential. Therefore, the improvement in the fixing property and color reproducibility of only certain colors is not effective, so the overlap and harmonization of the four color toners must be considered concerning the resin design and selection.
Because almost all colors can be reproduced theoretically by subtractive color mixing from three primary colors, i.e. yellow, magenta and cyan, the full-color copying machines in the current market operate by overlapping the three primary color toners. Therefore, although all tonalities can be realized in all density ranges in the ideal state, some areas for improvement still remain; for example, the spectroreflective and overlapping characteristics of the toners, the mixing properties during fixing, and the color saturation.
When obtaining black color by the overlap of the three primary colors, because three separate toner layers must be formed on the transfer paper, it is more difficult to improve the offset resistance compared with monochrome copying.
Further, the demand for high quality is further increasing concerning the full-color copied image. The ordinary customers, who have been used to seeing high quality color prints, are still not satisfied with full-color copy images, and require a quality very similar to prints or photographs, i.e. a solid image in a wider range of the copy image, homogeneous half-tone image, toners which provide high density images covering wider dynamic ranges, and transparent sheet images having a transparency similar to prints and transparency of the conventional toners.
The quickest and easiest method for satisfying these demands is to improve the dispersibility of the coloring agents existing in the toners. Japanese Patent Laid-Open Nos. 61-117565 and 61-156054 disclose methods, in which the toners are prepared by dissolving binder resins, coloring agents, and charge controlling agents into solvents and then removing the solvents. However, these methods have some problems; the difficult control of dispersibility of the charge controlling agents, and undesirable smell due to the solvents remaining in the formed toners.
A method for producing a toner by using halogenous solvents is disclosed in Japanese Patent Laid-Open No. 61-91666. However, the method has some drawbacks. For example, the usable coloring agents are limited due to the strong polarity of halogenous solvents.
Methods for producing toners in a kneader at a high temperature and pressure are disclosed in Japanese Patent Laid-Open Nos. 4-39671 and 4-39672. The methods are suitable for providing better dispersibility of coloring agents. However, molecular chains of the binder resins in toners are severed by excessive loading during mixing. As a result, a partial formation of low molecular weight polymer is promoted and high temperature offset readily occurs during the fixing process. In color copies particularly, because three or four fold color toner layers must be fixed, the latitude of the high temperature offset resistance is extremely restrictive compared with the monochrome toners so that only a small degradation of the polymer molecules readily causes high temperature offset.
In Japanese Patent Laid-Open No. 5-34978, the dispersion of a pigment into a resin is disclosed by feeding aqueous pressed cake of the pigment and resin and mixing with heat. The method provides desirable dispersibility of the pigment. However, the method does not mention the resin characteristics. The method differs from the present invention in the following points; a toner of the present invention has well balanced properties by using a resin designed so as to improve not only the fixing characteristics and offset resistance, but also dispersibility of the pigment. Thus, in the present invention, a desirable dispersion particle size of the pigment, good compatibility of the offset resistance and fixing characteristics, and improved color reproducibility can be achieved at the same time.